May 7, 2009
09:00 AM (EDT)

News Release Number: STScI-2009-08

Refined Hubble Constant Narrows Possible Explanations for Dark Energy

May 7, 2009: Less than 100 years ago scientists didn't know if the universe was coming or going,
literally. It even fooled the great mind of Albert Einstein. He assumed the universe must
be static. But to keep the universe from collapsing under gravity like a house of cards,
Einstein hypothesized there was a repulsive force at work, called the cosmological
constant, that counterbalanced gravity's tug. Along came Edwin Hubble in 1923 who
found that galaxies were receding from us at a proportional rate, called the Hubble
constant, which meant the universe was uniformly expanding, so there was no need to
shore it up with any mysterious force from deep space. In measuring how this expansion
was expected to slow down over time, 11 years ago, two studies, one led by Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University and Brian Schmidt
of Mount Stromlo Observatory, and the other by Saul Perlmutter of Lawrence Berkeley
National Laboratory, independently discovered dark energy, which seems to behave like
Einstein's cosmological constant.

To better characterize dark energy, Riess used Hubble Space Telescope's crisp view
(combined with 2003 data from NASA's Wilkinson Microwave Anisotropy Probe,
WMAP) to refine the value of the universe's expansion rate to a precision of three
percent. That's a big step from 20 years ago when astronomers' estimates for the Hubble
constant disagreed by a factor of two. This new value implies that dark energy really is a
steady push on the universe as Einstein imagined, rather than something more
effervescent (like the early inflationary universe) that changes markedly over time.